I was asked a question as to, a COMP-3 variable stores 18 digits using 10 bytes , i.e. S9(18).

Now the requirement changes and I am supposed to store S9(20), consider there is an increment in the length of the field. In this case what are the changes that I need to make? Rather, which storage will be best for S9(20)?

I had absolutely no idea, I have also searched across forums, but could not get any answer. Any help is appreiciated!

At the top of the page is a link to "IBM Manuals". The top set is for COBOL.

Suggest you look at the Language Reference for your version of cobol and read about numeric fields and their capacity. If you find something in the documentation that is not clear, post what you found and your doubt. Someone here will be able to clarify.